Heretofore there has been cutting inserts adapted to enhance chip discharge performance by a protruded portion protruding into a breaker groove. For example, the cutting insert described in Japanese Unexamined Patent Application Publication No. 6-190612 comprises a cutting edge formed along the periphery of the upper surface, a breaker groove formed inside of the cutting edge, a middle part lying more inward than the breaker groove, and a breaker projection (a protruded portion) protruding into the breaker groove from the middle part toward a corner.
The middle part has a breaker top face having a substantially polygonal shape when viewed from above, and the side face thereof is made up of a pair of breaker side faces shaped like an inclined surface extending from a breaker bottom face to the breaker top face. The breaker projection has a substantially trapezoidal shape when viewed from side, and is formed connectedly with the middle part so that the top face thereof is lower than the breaker top face.
In the cutting insert having the above shape, the breaker projection functions as a breaker when the depth of cut is small. When the depth of cut is large, the breaker top face and the breaker side face function as a breaker. When the depth of cut is medium, the breaker projection, the breaker top face and the breaker side face respectively function as a breaker.
However, the above cutting insert has the following problems (i) and (ii).
(i) The breaker side face is divided into right and left sides by a bisector of the corner. Therefore, under conditions where the depth of cut is small (especially a range in which the depth of cut is not more than a corner R) and feed rate is large, the polygonal tip end of the breaker top face contacts with the surface of a work material, and the chip discharge direction is unstable.
(ii) There is a small level difference between the breaker top face and the breaker projection. Therefore, the chips passing through the breaker projection under the conditions where the depth of cut is small and the feed rate is large run onto the breaker top face. This leads to loss of the function as a breaker, thus discharging irregular chips.
On the other hand, the cutting insert described in Japanese Unexamined Patent Application Publication No. 9-38808 comprises at the middle part. The middle part comprises a breaker top face whose corner has a substantially circular arc shape when viewed from above. The side surface of the middle part is made up of a breaker side face formed in the shape of an inclined surface extending from a breaker bottom face toward the breaker top face.
A breaker projection protruding into a breaker groove from the middle part toward the corner has an upward projecting curved surface shape and is formed connectedly with the middle part so that the top thereof is lower than the breaker top face.
In the cutting insert thus constructed, the breaker side face lying adjacent to the corner is formed in the curved surface, and hence an improvement in terms of the above problem (i) may seem to be achieved, however unsatisfactorily. Additionally, because the level difference between the breaker top face and the breaker projection is small, the same problem as the above (ii) may occur.